DIFFERENTIAL INCISION OF BEDROCK-CORED ANTICLINES IN THE 2016 KAIKŌURA EARTHQUAKE EPICENTRAL REGION, AOTEAROA NEW ZEALAND

Ongoing, oblique continental collision in Aotearoa New Zealand manifests in the northern South Island in an evolving transpressional plate boundary at the termination of the southern Hikurangi subduction zone. The 2016 Mw 7.8 Kaikōura earthquake highlighted the complexity of deformation associated with subduction terminations, producing a highly segmented and kinematically diverse multi-fault rupture at the southern margin of this subduction zone. Here we investigate how crustal shortening is distributed amongst active dip-slip faults bounding the margins of basement-cored anticlines in the epicentral region of the 2016 rupture. In this area of North Canterbury, northeast-southwest trending anticlinal ranges bounded by active reverse fault scarps accommodate a significant but unknown fraction of shortening associated with plate collision. Constraints on the slip rates and fault kinematics for these structures are generally unavailable based on a lack of chronology on displaced Quaternary geomorphic surfaces. New surficial geologic mapping based on airborne lidar reveals a suite of uplifted and differentially incised strath terraces present along the Waiau Uwha and Hurunui Rivers, which flow through these ranges and across the intervening synclinal basins. Incision of these strath terraces in these water gaps suggests substantial differential incision in response to ongoing bedrock uplift within folds, as well as tilting associated with ongoing folding across axial surfaces. Differential incision rates within the Lowry Peaks anticline approach ~0.6 mm/yr., based on regional correlation with existing Quaternary mapping frameworks. Additional chronologic constraints from luminescence dating of terrace cover sediments will refine these correlations and improve our estimates of differential incision driven by slip on the active, underlying reverse faults. Taken together, our results indicate that shortening associated with plate collision occurs within and on the boundaries of basement-cored anticlines in the Kaikōura epicentral region, and that slip rates on individual faults in this zone may approach ~1 mm/yr., in line or exceeding previous estimates based tectonic-geomorphic indices.